Rendering issues with fire and smoke in blender internal
I took a quick look at your Blend file and it looks like it might be your first attempt at fire in Blender Internal.  I made the following changes to get the Smoke portion of the Flamethrower effect you are working on to show:
  1) Your particle emitter / Flow control (plane obj) looks fine for now.  You can leave it as is.
  2) Your Smoke domain (cube obj) also looks fine for now - no changes needed.
  3) Your Material settings for the Smoke Domain are also okay.
  4) Your one Texture for Smoke requires two changes: First in the Influence section of the Texture Panel you need to check the "Volume Density" property and leave it set to one (1).  Second in the Mapping section of the Texture Panel you need to make sure that Generated is selected.  For some reason this defaults to NO selection that cause nothing to be displayed.  This is the minimal you must do to get the smoke to display when rendered.

Now you can run the animation in the 3D view, once smoke is being generated stop the animation and render the frame.  You should now see the smoke as a very white cloud.  This is a very minimal setting required to achieve a smoke effect.

The Blender Manual actually has a pretty good explanation of the proper settings for this Smoke/Flame effect using Blender Internal.

After you have the Smoke you will want to add a second texture to the same material making the changes to the properties as defined in the Manual.  Note there will be a LOT of tweeking to get the results you desire.

Best of luck.
TC